3 July 2016

Unid MFSK-4 160Bd 320Hz waveform


Quite strong signal heard for a long time on 9099.6 KHz (cf) on 02 July and recordered via the well known remote SDR receiver running at the Dutch University of Twente  (aka TwenteSDR): thus the strength of the signal must be intended as related to the Twente location.  
I came across this signal at 1252 UTC while I was watching two close (apparently) STANAG-4285 transmissions: zooming the FFT (fig. 1) I saw a weak MFSK-4 pattern so I tried to tune it via TwenteSDR in hopes of having a better signal... and I was right.

fig. 1 - the MFSK-4 signal as received at my QTH
Once analyzed, the signal spreads ~1300 Hz bandwidth and exhibits a 160 symbol/sec speed and 320 Hz as space bewteen tones (fig. 2): usually MFSK signals have the spacing equal to the speed of manipulation or the double, but not always.

fig.2 - MFSK parameters
A similar MFSK-4 signal, but running at 50Bd 100Hz space, was reported here.
Name and user of the waveform is unknown to me, I did some research and asked info to usually well-informed sources but unsuccessfully. Since the sensible differences bewteen the two receptions (see the screenshot at the top of the post  and figure 1) and the locations of the two receivers, it is reasonable to assume that if compared to my position (center Italy, Tuscany, locator JN52) the signal could come from the north, possibly north-east.

fig. 3
It's worth noting that the STANAG-4285 at 9098.6 KHz USB in the screenshot of fig.1, the French Navy S4285 circuit from Saissac, at that time was not received at TwenteSDR (or at least very very weak). 

I also tried a coarse demodulation of the signal using the SA FSK demodulator and directly mapping the output numbers 0,1,2,3 to the correspondents binary values 00,01,10,11 (fig. 4).
 
fig. 4
The bistream obtained in this way is shown in fig. 5: the analyzer finds a 102 bit length period, but although nuances of a frame structure are visible... it's juts a pictoric idea of the signal and it makes a poor sense.

fig. 5

No comments:

Post a Comment